The chemical attack is "a serious matter, it requires a serious response," Tillerson told reporters at a press conference in Florida. The top U.S. diplomat sketched out a long, difficult and potentially fruitless effort to push Assad from power.

"The process by which Assad would leave is something that I think requires an international community effort, both to first defeat ISIS within Syria, to stabilize the Syrian country, to avoid further civil war, and then to work collectively with our partners around the world through a political process that would lead to Assad leaving," he said. Pressed on whether he and Trump were working to assemble an international coalition to achieve that goal, Tillerson replied: "Those steps are underway."

The secretary of state also had tough words for Moscow, Assad's patron. "It is very important that the Russian government consider carefully their continued support for the Assad regime," he said.

And he reaffirmed what senior U.S. officials have said since the world first saw footage and photographs of gasping, dying or dead children in Khan Sheikhoun, a locality in the northwestern Syrian province of Idlib - Assad is to blame.

"There is no doubt in our minds and the information we have supports that Syria, the Syrian regime on the leadership of President Bashar al-Assad, are responsible for this attack," Tillerson said.

Trump contacted key lawmakers early Wednesday about Syria. Senate Armed Services Committee Chairman John McCain told Fox News that he had spoken by telephone with the president. "He's angry, as he well should be, and he's consulting with his military leadership as well as his secretary of state, and I have some optimism that he will take some concrete action here," the Arizona Republican said.

Later in the day, Trump held a joint press conference with Jordan's King Abdullah II and declared "I now have the responsibility" to lead the global response to the attack.

The frenzy of diplomatic and military planning capped a chaotic 10 days that began with Tillerson and the U.S. Ambassador to the United Nations, Nikki Haley, dropping longtime U.S. demands that Assad must go. And White House press secretary Sean Spicer told reporters that "there is a political reality we have to accept."